bronzes

Bronzes are traditionally composed of copper and tin; the resultant bronze alloys are harder than copper and wear-resistant. Also more fusible, bronze is hence easier to cast. Harder than pure iron, it is far more resistant to corrosion by seawater or moist air. An alloy of great historical interest, bronze is still wide employed. The first bronzes were produced more than five thousand years ago. The substitution of iron for bronze in tools and weapons from about 1200 BCE was the result of iron's abundance compared to that of copper and tin rather than any inherent advantages of iron. By the Middle Ages in Europe, certain proportions were known to yield specific properties. An alloy described in an 11th-century Greek manuscript in the library of St. Mark's, Venice, cites a proportion of one pound copper to two ounces of tin. This 8 to 1 ratio is approximately that used for bronze gunmetal in later times. Bell metal, characterized by its sonorous quality when struck, is a high-tin bronze alloyed with 20–25 percent tin. Besides the traditional use of bronze in weapons and tools, the alloy has also seen extensively use in coinage; most "copper" coins are actually bronze, typically containing about 4 percent tin and 1 percent zinc. Copper-based alloys containing manganese, iron, lead or zinc in greater proportion than tin may still be considered bronzes. Some modern bronzes substitute for tin such other metals as aluminum, manganese, and even zinc. These include aluminum bronze, silicon bronze, phosphor bronze and beryllium bronze. Certain bronze alloys contain small amounts of phosphorus, which improves the alloy’s hardness and strength, or zinc. Statuary bronze, containing less than 10 percent tin and an admixture of zinc and lead, is technically a brass. Up to 0.35% phosphorus added to bronze alloys containing up to 10% copper results in phosphor bronze, an alloy of great resilience, fatigue endurance, hardness and corrosion-resistance, is used for such applications as pump plungers, valves, and bushings. Manganese bronzes, in which there may be little or no tin but considerable amounts of zinc and up to 4.5 percent manganese, are useful in mechanical engineering. Aluminum bronzes, which can contain up to 16 percent aluminum and small amounts of other metals such as iron or nickel, are especially strong and corrosion-resistant. These are cast or wrought into pipefittings, pumps, gears, ship propellers, and turbine blades. Beryllium bronzes have good formability and high fatigue and yield strengths.